(Not Applicable)
1. Field of the Invention
This invention relates generally to metal detectors for detecting buried metal objects and more particularly relates to a metal detector using a ground piercing probe and associated electronic circuitry and methods for detecting not only the range of buried metal objects, but also the bearing and metal type of the buried metal object.
2. Description of the Related Art
Metal detectors have long been used by hobbyists as a favorable form of recreation, which offers not only an enjoyable activity, but also the opportunity to discover valuable and/or historical metal target objects buried in the soil.
In conventional prior art metal detecting, a user laterally reciprocates a metal detector, usually including a coil or other inductor, above a soil surface in a scanning pattern seeking an audible signal which indicates the presence, under the soil surface, of a metal object. The soil in which such buried metal objects are sought includes not only dirt and sand, and other manually movable or deformable earth surface materials, which are readily accessible to humans, but also includes underwater lake and sea bottoms.
A commonly used signal which indicates the presence of a metal object is an increase in the frequency of an audible tone. The typical user reciprocates the inductor of the typical metal detector above the surface in repetitive left and right, arcuate reciprocating movements until the frequency increase is heard or a tone is heard. Upon hearing a frequency increase, the user then begins reciprocating the inductor in smaller arcs across the region where the increased frequency signal was detected in an attempt to more precisely determine a position vertically above the buried metal object.
When permitted, a shovel is then used to dig up a clump of soil and the user then breaks apart and sifts through the clump using the user""s fingers or a tool in an attempt to find the metal target object. If the user is fortunate, the target object will be found in the clump of soil which has been dug up. Unfortunately, conventional metal detectors often cannot sufficiently accurately pinpoint the location of the buried metal object. Therefore, the metal detector is again employed and additional clumps of soil must be similarly dug up, sifted through and inspected. Also, the buried metal object may be located below the depth of the initial dig, necessitating deeper digging to retrieve the objects. Consequently, conventional metal detecting typically requires extensive manual labor for removing clumps of soil, breaking them apart- and sifting through them.
After completing these activities, a conscientious metal detector user will then carefully replace the soil in an attempt to return the soil as nearly as possible to its original condition. However, some users do not exercise such care, merely leaving a hole in the soil. Even users who carefully replace the soil have nonetheless created and left behind a substantial environmental disturbance in the soil.
These soil disturbances, especially at popular historical sites and highly trafficked outdoor public or park areas, often cumulatively cause both significant damage to the visual, cosmetic appearance of such areas as well as destruction of vegetation or other components of the local ecosystem and the creation of safety hazards.
As a result, many owners of private land and operators of public parks have imposed digging restrictions and regulations to minimize such damage or destruction. Typically, these restrictions limit the metal detector operator to digging in the soil with a tool no larger than a screwdriver and to digging only small holes to retrieve a metal target object. Because conventional metal detectors are insufficiently accurate to allow the target metal object to be located within a distance range tolerance of such a small hole, the operators use a probe, such as an ice pick or a screwdriver, to pierce the ground in the vicinity of the detected object in an attempt to strike the object and feel its presence. Such ground piercing does not cause significant soil disturbance and often is beneficial in providing aeration for trees or other vegetation. However, one problem with this technique is that it is a trial and error process which usually requires numerous ground piercings because such a probe only has a detection width equal to its own very narrow width. An additional problem is that such a probe and pierce technique does not enable the user to distinguish between the detected metal object and a nearby stone, tree root or other buried object which is harder than the surrounding soil. Consequently, small digs resulting in retrieval of only a stone or exposure or damage to a tree root, often occur. Furthermore, if an initial small hole has been dug and the object is not found, the problem remains for the user to determine in which direction from the hole the buried target object lies.
In an attempt to overcome the inherent inaccuracy of the conventional metal detector, the prior art has provided miniaturized metal detectors. These are of the same type as and modeled after the larger conventional metal detectors, but are much smaller. They are typically larger than xc2xdxe2x80x3 in diameter, have a plastic outer sleeve or shaft and use conventional metal detection circuitry. These miniature versions of conventional metal detectors are used to search for buried metal objects near the walls of a hole the user has dug or in dirt the user has dug out to form the hole and is sifting through.
There is therefore a need for a hand tool and associated electronic circuitry and methods to more precisely locate the buried metal target object after the general vicinity of its location has been detected by a conventional metal detector.
An object and feature of the present invention is that, after the general vicinity of a buried metal target object has been located by a conventional metal detector and before any holes have been dug, embodiments of the invention permit the location of the object to be more precisely detected, utilizing considerably fewer soil piercings without significant disturbance of the soil.
Another object and feature of the invention is to provide a metal detecting hand tool having a probe with sufficient strength, hardness and toughness so that it can survive repetitive insertions into undisturbed soil, especially hard soils and abrasive soils, such as sand.
A further object and feature of the invention is that each time the soil is pierced by the probe of the invention, the bearing of the object may be detected to guide the user toward the next most appropriate place to again pierce the ground, thus greatly improving the probability that the metal object will be struck by the next ground piercing.
A further object and feature of the invention is to detect not only the range and bearing to the buried metal object, but to detect information about the type of metal in that object. This permits the user to discontinue the effort to retrieve the metal object if the user is not interested in objects of the detected metal type and also permits the user to spend more time being more persistent if a potentially valuable metal is detected.
The invention is a method for detecting the metal type of a metal target object buried below the surface of a soil. The method begins by attempting to induce eddy currents in a target object by applying a current pulse to a coil which is coupled to a damping load. The current induced in a coil by the eddy currents is detected at a first sampling time. The first sampling time is located within a time interval beginning after termination of the current pulse applied to the coil. The time interval ends before the time at which the coil current, in the presence of a high conductivity metal object, decays to a current which exceeds the current to which the coil decays in the absence of a metal target object. Changes in the detected coil current are monitored and signaled as the probe is moved. These signaled changes in the coil current sample magnitude as the coil is moved closer to the metal object are then interpreted by the user to determine the type of metal object buried beneath the soil.